1. Field of the Invention
The present invention relates to a thermodilution catheter, and more particularly, to a thermodilution catheter having a flexible heating filament disposed therein for applying heat to the patient's blood for purposes of measuring cardiac output, volumetric blood flow, blood pressure, blood volume, blood components and the like.
2. Description of the Prior Art
As is well known, catheters have been developed for purposes of applying physiologic preparations directly into the blood streams of animals or humans or for measuring cardiovascular parameters such as cardiac output, blood pressure, blood volume, blood components and the like. Conventional catheters are made from various materials including plastics and are typically inserted into various body compartments, cavities and vessels to either deliver therapeutic agents, diagnostic agents, or to measure directly various physiologic parameters.
Numerous techniques have been disclosed in the prior art for measuring blood flow using catheters. For example, in U.S. Pat. No. 4,507,974, Yelderman describes a technique for measuring blood flow by applying a stochastic excitation signal to a system inlet and measuring the output signal at a downstream system outlet. The blood flow rate is then extracted by cross-correlating the excitation signal and the measured output signal. The problem addressed by systems of this type is particularly difficult since the physiologic blood vessels are elastic, thereby making classic fluid measuring techniques unacceptably inaccurate. In fact, because the blood vessels are elastic, blood flow cannot be measured unless (1) the physical heart dimensions are measured simultaneously with the blood velocity, (2) a technique is used which is independent of the vessel geometry or (3) a blood velocity technique is used which is calibrated by some other technique. Examples of each of these techniques may be found in the prior art.
For example, a prior art approach for simultaneously measuring blood velocity and vessel geometry is described by Segal in U.S. Pat. Nos. 4,733,669 and 4,869,263 and in an article entitled "Instantaneous and Continuous Cardiac Output Obtained With a Doppler Pulmonary Artery Catheter", Journal of the American College of Cardiology, Vol. 13, No. 6, May 1989, Pages 1382-1392. Segal therein discloses a Doppler pulmonary artery catheter system which provides instantaneous diameter measurements and mapping of instantaneous velocity profiles within the main pulmonary artery from which instantaneous cardiac output is calculated. A similar approach is taught by Nassi et al. in U.S. Pat. No. 4,947,852. A comparable ultrasound technique is disclosed by Abrams, et al. in U.S. Pat. Nos. 4,671,295 and 4,722,347 and in an article entitled "Transtracheal Doppler: A New Procedure for Continuous Cardiac Output Measurement", Anesthesiology, Vol. 70, No. 1, January 1989, Pages 134-138. Abrams et al. therein describe a technique whereby a piezoelectric ultrasound transducer is placed in the trachea of a patient in proximity to the aorta or pulmonary artery so that ultrasound waves may be transmitted toward the path of flow of blood in the artery and reflected waves received. The cross-sectional size of the artery is measured based upon the Doppler frequency difference between the transmitted and received waves. Imaging techniques such as x-ray or radio isotope methods have also been used.
Previous techniques which are geometry independent include an indicator dilution or dye dilution technique of the type first disclosed by Stewart in an article entitled "The Output of the Heart in Dogs", American Journal of Physiology, Vol. 57, 1921, Pages 27-50. Other such geometry independent techniques include a thermodilution technique as first described by Fegler in an article entitled "Measurement of Cardiac Output in Anesthetized Animals by a Thermo-Dilution Method", Quarterly Journal of Experimental Physiology, Vol. 39, 1954, Pages 153-164 and an ionic dilution technique as described by Geddes et al. in U.S. Pat. No. 4,572,206.
On the other hand, prior art techniques for measuring blood velocity which require a secondary calibration technique include a pulse contour technique of the type described by Schreuder, et al. in an article entitled "Continuous Cardiac Output Monitoring During Cardiac Surgery", Update In Intensive Care And Emergency Medicine, Berlin: Springer-Verlag, 1990, Pages 413-416. Another so-called "hot wire" anemometer or heated thermistor technique has been described, for example, by Tanabe, et al. in U.S. Pat. No. 4,841,981 and EP 235811 and by Sekii, et al. in U.S. Pat. No. 4,685,470 and WO 8806426.
The present invention relates to a geometry independent technique, namely, indicator dilution. In conventional indicator dilution techniques, different methods of indicator delivery may be used. For example, Khalil in U.S. Pat. No. 3,359,974 introduces indicator as a step increase and measures the resultant distal temperature change. Newbower, et al., on the other hand, discloses in U.S. Pat. No. 4,236,527 the technique of introducing the indicator as a sinusoid and measuring the distal wave attenuation. In addition, the indicator may be applied as an impulse so that the area under the resultant response may be measured as described by Normann in U.S. Pat. No. 4,576,182. Eggers, et al. in U.S. Pat. No. 4,785,823 similarly provide an impulse, but Eggers, et al. use high frequency energy to provide large heat fluxes to the blood without increasing the filament temperature. In addition, Petre describes in U.S. Pat. No. 4,951,682 an intra-cardiac impedance catheter which measures cardiac output based on changes in the electrical impedance of the blood in the right ventricle. By contrast, as described by Yelderman in the afore-mentioned U.S. Pat. No. 4,507,974, the indicator may be supplied according to a pseudo-random binary sequence and the distal response measured. Cross-correlation can then be performed between the input sequence and the output sequence, and flow is computed based upon the area under the cross-correlation curve. Each of these techniques may provide either an intermittent or a continuous measurement.
Although each of the above-mentioned techniques may use a variety of indicators, heat is the preferred indicator to be used in the clinical environment, for unlike other indicators, heat is conserved in the immediate vascular system but is largely dissipated in the periphery in one circulation time so as to eliminate recirculation and accumulation problems. On the other hand, if cold (negative heat) indicators are used, large amounts of cold may be used, for cold has relatively no deleterious effects on blood and surrounding tissues. However, when cold is used, it must be supplied in a fluid carrier such as saline since cold producing transducers are not readily economical or technically available at present. For example, such a technique is described by Webler in U.S. Pat. No. 4,819,655 and by Williams in U.S. Pat. No. 4,941,475, but the cold-based technique of Webler or Williams has significant clinical limitations in that the circulating fluid must be cooled to near ice temperature prior to input into the catheter and temperature equilibrium established, which takes a significant amount of time. In addition, the enlarged catheter segment containing the cooling elements may restrict blood flow. By contrast, if heat is used, a maximum heat infusion limitation is quickly reached since small increases in heat transducer temperature can have a deleterious effect on blood and local tissue. In fact, it can be inferred from the teachings of Ham et al. in "Studies in Destruction of Red Blood Cells, Chapter IV. Thermal Injury", Blood, Vol. 3, pp. 373-403 (1948), by Ponder in "Shape and Transformations of Heated Human Red Cells", J. Exp. Biol., Vol. 26, pp. 35-45 (1950) and by Williamson et al. in "The Influence of Temperature on Red Cell Deformability", Blood, Vol. 46, pp. 611-624 (1975), that a maximum safe filament surface temperature is probably about 48.degree. C. Since the surface temperature of a heat transducer is a function of the blood flow velocity, the surface area and the heat flux, the optimum design is to maximize the heat delivered to the blood while minimizing the transducer surface temperature.
A heat transducer must satisfy several requirements if it is to be used clinically. Namely, the heat transducer or filament must be electrically safe. It also must only minimally increase the catheter cross-sectional area or diameter and must be made of materials which are non-toxic, which can be sterilized, and which can safely and easily pass through a standard introducer sheath. The heater must also be flexible so as not to increase the stiffness of the catheter body and must be capable of transferring the electrically generated heat to the surrounding blood without exceeding a safe filament surface temperature. Moreover, means must be present to continuously monitor the filament temperature to detect unsafe filament temperature and/or stagnant blood flow. However, prior art heater elements for catheters have not heretofore addressed these problems.
Prior art heater elements for thermodilution catheters have typically used simple resistive wire wound around the catheter. For example, Khalil discloses in U.S. Pat. No. 3,359,974 and Barankay, et al. disclose in an article entitled "Cardiac Output Estimation by a Thermodilution Method Involving Intravascular Heating and Thermistor Recording", Acta Physiologica Academiae Scientiarum Hungaricae, Tomus 38 (2-3), 1970, Pages 167-173, wrapping the wire around the catheter but describe no methods for securing the heater material to the catheter body. Normann, in U.S. Pat. No. 4,576,182, discloses a design similar to that of Khalil. However, such exposed or minimally required wire as used in these devices increases the catheter cross-section, thereby making it difficult for the catheter to pass through an introducer and providing a rough surface which may introduce local blood clot formation. In addition, such an arrangement provides no protection from fragments of the filament becoming dislodged and does not evenly dissipate the heat, thereby producing "hot" spots near the filament itself. It is thus desired to design a catheter heating filament which is electrically, mechanically and thermodynamically safe.
As noted above, although heat is a preferred indicator for dilution methods for measuring blood flow, the amount of heat delivered is limited or restricted by the maximal safe filament surface temperature. Although no absolute safe maximum filament temperature has been developed in the prior art, sufficient information is present in the literature to substantiate a reasonably safe maximum. For example, amongst blood, proteins and vessel tissue, red blood cells have been shown to be probably the most susceptible to higher temperatures. It is also well documented in the prior art that red blood cells can sustain an incubation temperature of 48.degree. C. for up to one hour before developing significant abnormalities, as described by Williamson, et al. In any event, because the actual contact time of each red blood cell flowing past the heating filament is significantly less than several seconds, such a maximum of 48.degree. C. is easily acceptable. A catheter heating filament can be designed to provide sufficient surface area to allow adequate heat infusion with a surface temperature below this maximum; however, changes in flow, such as sudden decreases or cardiac arrest, or changes in catheter position, such as becoming lodged against a vessel wall, may provide a local blood environment. Such stagnant environments may allow for surface temperatures which exceed these maximum limits and can thus cause harm if a method is not present for measuring filament temperature.
In addition, in prior art heating filaments for thermodilution catheters either the filament temperature has not been measured or the temperature is measured with a second thermometer. Such techniques obviously are unacceptable if a maximum safe temperature is to be maintained. Acceptable temperature sensing materials require a sufficiently high temperature coefficient of resistance to measure changes in temperature. Compositions of this type are described, for example, by Morris, et al. in an article entitled "Thin Film Thermistors", Journal of Physics Engineering: Scientific Instruments, Vol. 8, 1975, Pages 411-414. It is desired to develop a method for continuously measuring the filament temperature without the use of a secondary measuring transducer such as a thermistor or thermocouple of the type used in these prior art devices.
A classical prior art method of measuring fluid velocity uses a hot-wire anemometer. In accordance with this technique, a filament is heated with a constant power or at a constant heat flux and the resistance is measured. If a filament with a high temperature coefficient of resistance is used, the measured resistance can be used to directly calculate filament temperature, for the filament temperature is monotonically and inversely proportional to the fluid velocity. Such a technique has been previously described as a means for measuring blood velocity by Gibbs in an article entitled "A Thermoelectric Blood Flow Recorder in the Form of a Needle", Proc. Soc. Exp. Biol. & Med., Vol. 31, 1933, Pages 141-146. However, as noted by Gibbs in that article, such a technique has been limited to peripheral vessels and cannot give absolute blood volumetric flow rates, only velocity. It is desired to adapt such techniques to thermodilution measurements to prevent localized overheating of the blood.
As noted above, previous heating elements for thermodilution catheters have generally used wire and wrapped it around the catheter. However, such an approach provides for uneven heat densities on the catheter since the wire tends to be hot and the space between the wire cooler. A more uniform material is desired which allows for more even heat densities and the elimination of "hot" spots. This is not possible with wire, for wire, even very small gauge wire, provides a larger cross-section for the catheter than necessary since there is unused space between the wire even when the wire is wound very compactly. A more uniform material would allow for a better distribution of the same quantity of heat with only a small increase in catheter cross-section. It is thus desirable to develop a filament material which minimally increases a cross-sectional area of the catheter and which provides a more uniform filament heat flux.
Accordingly, there are numerous problems with prior art thermodilution catheters which render them either inaccurate or unacceptably unsafe for use in the clinical environment. There is thus a long-felt need in the art for a filamented thermodilution catheter which overcomes these limitations of the prior art so as to allow production of a safe, accurate thermodilution catheter. The present invention has been designed to meet this need.